This invention relates generally to improvements in folding tubular frames of the type commonly used in folding wheelchairs. More particularly, this invention relates to an improved pivot joint assembly for pivotally interconnection tubular members of a folding wheelchair frame, wherein the improved pivot assembly has a simplified and economical construction designed facilitated assembly and extended service life.
Folding wheelchair frames are well known in the art to provide a wheelchair construction adapted for folding movement between a collapsed compact configuration for shipment and storage, and an expanded configuration for normal use. Such wheelchair frames are commonly formed from metal tubing to provide lightness in weight together with relatively high structural strength, thereby providing a wheelchair which can be handled and maneuvered relatively easily yet provides a prolonged service life. However, to achieve the desired frame folding capability, movable joint structures are required to interconnect various tubular frame members, wherein these movable joint strucutres must be designed to accommodate easy frame folding over a prolonged chair life yet possess substantial structural strength to withstand the frequently large mechanical loads applied to the frame during wheelchair use. Unfortunately, these requirements have resulted in the use of complicated and costly joint structures designed to avoid mechanical failure, or, alternately, the use of compromise joint designs of reduced cost but increased likelihood of early mechanical failure.
More particularly, by way of one specific example, many folding wheelchair frames include a pair of side frames having the wheelchair wheels mounted thereon and a foldable seat and seatback connected therebetween. The two side frames are interconnected by a central cross brace of typically X-shaped configuration defined by two centrally pivoted tubular cross brace members each interconnected between tubular frame members of the two side frames. Importantly, at least one end of each cross brace member is connected in a T-shaped configuration to the adjacent side frame member by a pivot joint permitting the cross brace member to pivot or rotate about a central axis of the side frame member during folding movement of the wheelchair chair between folded and unfolded positions. In the past, this pivot joint has typically been constructed by welding the end of the cross brace member generally at a right angle to an outer pivot sleeve which is then rotatably mounted about an inner support tube connected in-line with the side frame member. While this pivot joint construction advantageously provides high structural strength for extended service life, a relatively large number of joint components and manufacturing steps are required to result in a relatively costly overall wheelchair frame construction. Moreover, it has been found that moisture can become trapped between various components of the pivot joint, particularly such as between the outer pivot sleeve and the inner support tube, resulting in undersired rust-out and failure of the pivot joint.
Accordingly, there exists a significant need for an improved pivot joint assembly for use in folding tubular frames of the type used in wheelchairs and the like, wherein the pivot joint is formed from a small number of easily assembled components adapted to provide high structural strength and easy frame folding during use. The present invention fulfills these needs and provides further related advantages.